EXTRINSIC CONTROL OF STEM-CELL FATE - PRACTICAL CONSIDERATIONS

Decades of experimental data suggest that hematopoietic stem cells can remain quiescent, divide, differentiate or die and further, that thes e cell fate decisions are determined by extracellular signals provided by the hematopoietic microenvironment (ME). Given the importance of t he ME for regulating hematopoiesis, it is imperative that transplanted stem cells migrate efficiently and home to appropriate niches where t hey can receive regulatory signals. Currently the rapid engraftment se en after transplantation of cytokine-mobilized blood-derived stem cell s would suggest that these cells are well-equipped for homing. More re cent concerns have now been raised by the possibility that in vitro ex pansion of these stem cells may diminish their ability to engraft. One possible explanation for this is that expansion protocols may alter a dhesion molecule expression and consequently homing. Data presented in this report indicate that expression of adhesion molecules is altered following in vitro exposure to recombinant cytokines, and that variou s combinations of cytokines differentially modulate adhesion molecule expression.